The invention relates to a method and apparatus for effecting screening of medium consistency suspensions, particularly wood pulp stock having a consistency in the range of about 6-15 percent solids. When handling wood pulp stock, it is necessary on some ocassions to remove shives, dirt, and the like from the pulp. It is highly desirable if this screening can be done without dilution of the pulp. Dilution, and subsequent re-thickening, requires additional expensive equipment, and a relatively high energy penalty.
According to the present invention medium consistency wood pulp, and like suspensions, can be screened to remove the shives, dirt, and other reject material therefrom, without the necessity of dilution and then re-thickening. These desirable results can be achieved by utilizing a simple, relatively inexpensive piece of equipment which has long life and effectively performs the screening function.
According to the method of the present invention, screening of a suspension of comminuted cellulosic fibrous material, or a like suspension, having a consistency of about 6-15 percent solids, preferably about 8-14 percent solids, to separate reject material from the suspension is practiced. A substantially flat plate screen mounted in a chamber having a suspension inlet, a suspension outlet on the opposite side of the screen as the suspension inlet, and a rejects outlet is utilized. The method comprises: (a) Passing the suspension into the chamber under pressure. (b) Effecting fluidization of the suspension at the screen area, while maintaining the suspension under pressure. (c) Causing the fluidized suspension to ultimately pass either through the screen to the suspension outlet, or towards the rejects outlet; and (d) Continuously effecting passage of the suspension passing through the screen through the suspension outlet, and the reject material through the rejects outlet.
Fluidization is preferably accomplished by utilizing a disk mounted for rotation adjacent the screen, and rotating the disk at high speed (e.g. 1,500 rpm). The suspension is preferably passed into the chamber along the axis of rotation of the disk, the screen being annular and surrounding the inlet. The outlet for the suspension, and the rejects outlet, may both be radially extending.
A second chamber, with like components, may be provided connected to the first chamber. In such a case, the rejects outlet from the second chamber is provided in communication with the suspension inlet to the first chamber, and the suspension outlet from the first chamber comprises the suspension inlet to the second chamber. A plurality of additional chambers may also be provided if desired.
Exemplary apparatus according to the present invention includes an annular substantially flat plate screen mounted within a closed chamber. An inlet to the chamber is substantially concentric with the screen, and a suspension outlet from the chamber is on the opposite side of the screen from the inlet. A disk comprising first and second faces is mounted so that it is substantially parallel to the screen, and only slightly axially spaced therefrom. The first face of the disk has a plurality of vanes mounted thereon and extending generally radially therefrom, each of the vanes including an end portion disposed radially outwardly from the disk periphery. A rejects outlet extends radially outwardly from the chamber, adjacent the periphery of the disk.
It is often desirable to provide recirculation of a certain amount of the rejects, to reduce the consistency gradient from the suspension inlet to the rejects outlet. This can be done by providing a plurality of openings in the disk adjacent the center thereof, the openings disposed in-line with the suspension inlet. A plurality of impeller blades may be mounted on the second face of the disk, extending generally radially and contained within the periphery of the disk to facilitate recirculation.
It is the primary object of the present invention to provide for the effective removal of shives, dirt, and like reject material from comminuted cellulosic fibrous material suspensions, or the like, of medium consistency. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.